1. Technical Field
The present invention relates generally to heat exchangers in turbofan gas turbine engines and, more specifically, to heat exchangers and regulating flow in fan bypass ducts of the engines.
2. Background Information
At least some known aircraft gas turbine engines include a fan, a compressor, a combustor, a high pressure turbine, a low pressure turbine, and an augmentor or “afterburner” and an exhaust nozzle. The compressor, combustor, high pressure turbine, and low pressure turbine are collectively referred to as a core engine or engine core.
Airflow entering the fan is compressed. Airflow exiting the fan is split such that a portion of the flow referred to as core engine flow is directed into the compressor and the remaining portion of the airflow, referred to as fan bypass flow, is directed into a bypass duct or passage where it bypasses the compressor, the combustor, the high pressure turbine, and the low pressure turbine. Airflow entering the compressor is compressed and directed to the combustor where it is mixed with fuel and ignited, producing hot combustion gases used to drive both the high pressure and the low pressure turbines. Moreover, at least some known gas turbine engines combine a portion of the fan bypass flow with the airflow exiting the low pressure turbine forming an exhaust flow. The exhaust flow may be further heated in the augmentor before exiting through the exhaust nozzle.
Variable cycle or variable bypass gas turbine engines have been designed to combine high thrust capabilities of turbojets with good fuel efficiency of turbofan engines. Typically in variable cycle engines, the amount of air that is bypassed is changed to suit aircraft speed.
The bypass air is often modulated or regulated by various devices for various reasons. To regulate an amount of bypass air supplied to the augmentor, at least some gas turbine engines include a valve assembly. More specifically, in some known gas turbine engines, the fan bypass flow is regulated based on specific exhaust liner pressure ratio requirements demanded for the type of flight mode of the aircraft.
Variable cycle systems have been considered for use in typical military engines that use augmentors (afterburners) to provide additional thrust at supersonic speeds. Afterburning turbofan engines typically utilize mixers that take part of the engine's bypass air and mix or inject that air into the core engine flow in an engine's afterburning section. Typically, it is desirable to increase the total bypass flow at dry operating conditions and to reduce the bypass flow at augmented conditions. Under dry conditions, the object is to improve specific fuel consumption and during augmented conditions, the object is to improve thrust.
Rear Variable Area Bypass Injectors (rear VABI's) are used to inject the bypass air at the afterburner and forward Variable Area Bypass Injectors (forward VABI's) are used to inject or control bypass air flowing into the fan bypass duct. Some examples of such VABI's are described in various U.S. patents including U.S. Pat. No. 4,069,661; U.S. Pat. No. 4,064,692; U.S. Pat. No. 4,072,008; U.S. Pat. No. 4,010,608; U.S. Pat. No. 4,068,471, and U.S. Pat. No. 4,175,384.
VABI's and other types of valves used to regulate an amount of bypass air supplied to the augmentor may include a plurality of adjustable or variable blocker doors or variable vanes. Variable vanes disposed in fan ducts of aircraft high bypass gas turbine engines and FLADE engines are two examples of such an apparatus. U.S. Pat. No. 7,758,303, issued Jul. 10, 2010, entitled “FLADE Fan With Different Inner And Outer Airfoil Stagger Angles At A Shroud Therebetween” discloses a variable FLADE inlet guide vanes disposed in a FLADE duct which surrounds a core engine. U.S. Pat. No. 4,080,785, issued Mar. 28, 1978, entitled “Modulating bypass variable cycle turbofan engine” discloses flaps at downstream ends of fan bypass ducts for variable area fan nozzles. U.S. Pat. No. 7,721,549, issued May 25, 2010, entitled “Fan variable area nozzle for a gas turbine engine fan nacelle with cam drive ring actuation system” discloses a fan variable area nozzle including a flap assembly which varies a fan nozzle exit area. U.S. Pat. No. 4,292,802, issued Oct. 6, 1981, entitled “Method and apparatus for increasing compressor inlet pressure” discloses a plurality of blocker door vanes disposed in the bypass duct to selectively close off the bypass flow and increase the flow and pressure of the air flowing into the compressor.
Two regulating valves are disclosed in U.S. patent application Ser. No. 11/753,929, filed May 25, 2007, entitled “METHOD AND APPARATUS FOR REGULATING FLUID FLOW THROUGH A TURBINE ENGINE” and Ser. No. 11/753,907, filed May 25, 2007, entitled “TURBINE ENGINE VALVE ASSEMBLY AND METHOD OF ASSEMBLING THE SAME”. These regulating valves includes an outer fairing coupled to a radially outer duct wall, an inner fairing coupled to a radially inner duct wall, and a translatable annular slide valve. The annular slide valve is selectively positioned between the fairings such that at least one flow area between the slide valve and the fairings is varied. The VABI's, variable vanes, and other types of valves are referred to herein as variable geometry flow restrictor.
Modern gas turbine engines and variable cycle engines require cooling air or other fluids for the hot components such as turbine components and aircraft avionics. Other type of heat exchangers found in gas turbine engines are used for cooling oil, fuel and water. This cooling air or fluid often requires a heat exchanger to transfer energy into the bypass of the engine and the heat exchanger is often located in the bypass duct or uses bypass duct airflow. Variable cycle engines can provide large efficiency and performance boosts over a large range of power requirements. The VABI's and blockers, generally referred to herein as bypass valves, have been developed for turbofan aircraft engines to take advantage of these benefits.
Heat exchangers in the bypass duct and bypass valves often conflict with one another, especially if they use the same air. Heat exchangers restrict airflow and hurt the engine performance benefit of a variable geometry engine (an engine using bypass valves). Variable geometry often restricts the heat exchangers flow when it is needed most.
Accordingly, it is desired to provide a gas turbine engine having a heat exchanger cooled by fan bypass flow and a bypass valve for regulating or restricting the fan bypass flow. It is further desired to have low or reduced performance conflicts between the heat exchanger and bypass valve.